Something awkward to work out to -- that's horrid, that is! (7)
I believe the answer is:
toughie
'something awkward to work out' is the definition.
(eg a difficult question)
'to that's horrid that is' is the wordplay.
'that's horrid' becomes 'ugh' (exclamation of disgust).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'to'+'ugh'+'ie'='TOUGHIE'
(Other definitions for toughie that I've seen before include "A difficult test" , "DT's new difficult crossword" , "poser" , "Yob" , "Difficult problem" .)